Heat treated cast iron and process of producing the same



July 16, 193.5- M. G. JEwETT Er AL. 2,008,452

HEAT 'fREATED CAST IRON ND PROCESS OF PRODUCING THE SAME Filed sept. 23,

, f mi.

BY .Sama/e5 arraly,

A TTORNE Patented July 16, 1936 UNITED STATES.

PATENT oFFicE HET TREATED CAST IRON AND PROCESS OF PRODUCING THE SAME Maurice G. Jewett and Samuel asslgnors to Chain Belt Milwaukee, Wis.,

C. Harris,

Company, Milwaukee, Wis., a corporation of Wisconsin Application september z3, 1931, serial N6. 564,634r

ez claims. (ci. 14e-21.7)

strength, yield point, and resistance to wear;

and is a continuation inv part of our application, led September 17, 1930, Serial Number 482,589. An object of the invention is to modify the character of the iron-carbon grain composition 0 by appropriate heat treatment, in order to 4dethe commercial malleable cast iron obtainable in the open market is considerably more ductile than unmalleabilized cast iron, at the same time In the accompanying drawing, the photomicro- .V

graphs are all made at the same magncation, 1000 diameters.

Fig. 1 is a copy of a photomicrograph of a polished and etched surface or ordinary malleable cast iron;

Fig. 2 is a copy of a photomicrograph of a polished and etched surface of malleable cast iron after having been subjected to a heat treatment of about 1600" F. for onehour, and immediately drastically cooled, showing amartensitic grain structure;

Fig. 3 is a copy of a phctomicrograph of a polished and etched surface of malleable cast iron after having been heat treated to about 1600A F. for one hour, immediately drastically cooled, and again heat treated or tempered at 1325 F. for one hour, and immediately drastically cooled, and

showing a homogeneous spheroidized cementite grain structure; and

Fig. 4 is a copy of a photomicrograph of a polished and etched surface of a commercial type of malleable cast iron after having been heat treated to about 1475" F., then quenched, then reheated to from -900 F. to 1200 F., and again quenched; the illustration exhibits a pearliticsorbitic grain structure.

In Fig. 1, Vwhich illustrates the physical structure of regular commercial malleable cast iron as produced at the present time, the pure ferrite crystals l are illustrated as having thin boundary lines, 2, and also illustrates inclusions, or islands of graphitic carbon 3. Upon heating such cast iron to well above its critical or carbon combining pointffor example, from about 1475" F. to about 1600 F.' or higher, which may be in the presence of a carburizing agent to increase the carbon content of the outside surface of the iron which has been previously decarburized, for a suilicient length of time, about one hour or longer depending upon the mass of the iron, to allow the ferrite grain to absorb or combine with variable quantities of carbon from the associated graphite and form an iron carbide, for example, from about one-tenth percentto about one percent 'of carbon, and then immediately drastically quenching, by immersion in oil or water to a point below about 900 F., or very materially below its carbon combining temperature, the ferrite grains will combine with carbon and will have been converted into a martensitic Structure as shown at 4 in Fig. 2, and the product will be very wear-resisting. The length of time the metal is held 4at a temperature above the critical point will determine the percentage of graphitic carbon' combined with the ferrite, and the characteristics of this martensitic structure will be controlled by the percentage of carbon absorbed by the ferrite crystals. Disregarding the inclusions, or islands of graphite 3, the cast iron thus treated will have a structure corresponding to that of a carbon steel of like perture illustrated in Fig. 2, to a temperature from about 1300 F.. to about 1325 F., or slightly higher, for about one hour, (which is just below the critical point range, about 1350 F. to about 1450 F.) and then immediately drastically quenching in oil to about 900 F. or lower, the

iron will possess the greatest ductility with the maximum increased strength, and will have its grain composition converted into a homogeneous spheroidized cementitepearlitic structure 'as illustrated at 5, in Fig. 3, but not into the sorbitic structure illustrated in Fig. 4; and disregardlng 4the graphite inclusions, the structure illustrated in Fig. 3 resembles the analogous pearliticspheroidal-cementite structure that is exhibited by steel of like carbon content produced in the regular commercial way.

A known grade ofiron, produced by heating malleable cast iron to about l475 F. for a sumcient time, and then quenching in oil, water, or other medium, and then reheating to from 900 F. to about 1200* F. produces a producthaving a grain structure as illustrated in Fig.-4, in which the martensitic structure has been converted into a pearlitic-sorbitic structure shown at 6. The iron produced by the treatment in acccrdance with the present invention, by a rst heating to about 1600 F., then immediately drastically cooling, and then a second heating to 1325'l F. and immediately drastically cooling, does not produce a grain having a sorbitic structure, but on the contrary produces a grain having a homogeneous spheroidized cementite pearlitic structure illustrated at 5 in Fig. 3, as above noted which has about twenty percent greater strength than the pearlitic-sorbitic structure, with a corresponding increase in yield point. and a combined carbon content from about 0.50 percent to about 0.85 percent, a Brinell hardness from about to about 217, a tensile strength of 'from Heat to 1475 F., and drastically quench in oil;

Reheat to 13.25 F., and drastically quench in oil;

Combinedcarbon .48%

Brinell 143 l Tensile strength 75,650 pounds per sq. in.

Elongation 13% Heat to 1525 F., and drastically quench in oil;

Reheat to 1325 F., and drastically quench in oil;

Combined carbon .54%

Brinell 149 Tensile strength 84,200 pounds per sq. in.

Elongation 9% Heat to 1600 F., and drastically quench in oil;

Reheat to 1325 F., and drastically quench in oil;

Combined carbon .82%

Brinell 217 Tensile strength 90,000 pounds per sq. in.

Elongation 5% The treatment of malleable cast iron by the processes of the present invention produces a product having the characteristics indicated in -the examples, given above, havinga combined carbon content of 1rom'0.50% to 0.85%, and a Brinell hardness of 145 and upwards with excellent machining qualities, and with an ultimate strength and yield point of from about '75,000 pounds to about 87.000 pounds, and upwards, per squareinch. l

Mallcable iron, as the term is used in this speciilcation and claims. refers to any cast iron which has the property of malleability; one example thereof being where such property, as an end result, has been developed through heat treatment of a metal such as the so-callcd white iron, which in most cases has no appreciable malleability prior to such heat treatment. However, as in ordinary commercial. usage, such cast irons are to be clearly distinguished from steel, which is also an iron product possessing malleability. The present claims are not intended to extend to this latter class of iron products, but only to cast irons as that term is ordinarily understood in the art.

While for purposes of the present disclosure whereby others maybe enabled to practice the invention, one particular example has been described, it will be obvious that those skilled in the art to which it appertains may vary the details thereof Without departing from the gist oi the invention, which is the production of a heat treated cast iron having a homogeneous spheroidized cementite matrix structure. with the major portion of the carbon being in the graphitic form; and therefore it is not wished to be limited to vthe above disclosure except as may be required by the appended claims.

1. The method of developing in malleable cast iron, a grain having a homogeneous spheroidized cementite pearlitic structure, substantially free from sorbitic structure, which comprises subjecting malleabilized cast iron to a temperature from about 1475 F. to about 1600 F. for a time sumcient to combine with associated carbon, then drastically cooling to form a martensitic structure, then reheating to about 1325" F. for a time suilcient to convert said martensitic structure into a homogeneous spheroidized cementite pearlitic structure substantially free from sorbitic structure, and again drastically cooling.

2. The method of developing in malleable cast iron, a grain having a homogeneous spheroidized' cementite pearlitic structure, substantially free from sorbitic structure, which comprises subjecting malleabilized cast iron to a temperature from about 1475 F. to about 1600 F. for not less than one hour, then drastically cooling to form a martensitic structure, then reheating to about 1325 F. for not less than one hour to convert said martensitic structure into a homogeneous spheroidized cementite pearlitic structure substantially free from sorbitic structure, and again drastically cooling.

3. The method of developing in malleable cast iron, a grain having a homogeneous spheroidized cementite pearlitic structure, substantially free from sorbitic structure, and developing in the treated iron a strength of 87.000 pounds and upwards per square inch, which comprises subjectlng malleabilized cast iron to a temperature of about 1600 F. for a time suilicient to combine with associated carbon, then drastically cooling to form a martensitic structure, then reheating to about 1325 F. for a time sufficient to convert said martensitlc structure into a. homogeneous spheroidized cementite pearlitic structure substantially lil of about 1600 F. for not less than one hour, then drastically cooling to form a martensitic structure, then 'reheating to about 1325 F. for not less than one hour to convert said martensitic structure into a homogeneous spheroidized cementite 5. The method of developing in malleable cast iron, a grain having a homogeneous spheroidized cementite pearlitic structure, and having a combined carbon content of from about 0.50 percent to about 0.85 percent, which comprises subjecting malleabilized cast iron Ato 'a temperature: of about 1600 F. for a-time sunicient to combine with associated carbon, then drastically cooling to form a martensitic structure, then reheating to from l300 F. to about`1325 F. for a time suflicient to convert said martensitic structure into a homogeneous spheroidized cementite pearlitic structure, and again drastically cooling.

6. The method of developing in malleable cast iron, a grain having a homogeneous spheroidized` cementite pearlitic structure, substantially free from sorbitic structure, and having a combined carbon content of from about 0.50 percent to about 0.85 percent, which comprises subjecting malleabilized cast iron to a temperature of about 1600 F. for not less than oneA hour, then drastically cooling to form a martensitie structure, then reheating to about 1325 F. for not less than onel hour to convert said martensitic structure into a homogeneous spheroidized cementite pearliticV structure substantially Afree from sorbitic strucform a martensitic structure, then reheating to about 1325 for a time sufficient to convert said martensitic structure into a homogeneous spheroidized cementite pearlitic structure substantially free from sorbitic structure, and again drastically cooling.

8. The method of developing in malleable cast iron, a grain having a homogeneous spheroidized cementitepearlitic structure, substantially free from sorbitic structure, and having a combined carbon content of from about 0.50 percent to about 0.85 percent, which comprises subjecting malleabilized cast iron to a temperature of about 1600 F., in a carburizing environment, for not less vthan one hour, then drastically cooling to form a martensitic structure, then reheating to about 1325 F. for not less than one hour to convert said martensitic structure into a homogeneous spheroidized cementite pearlitic structure substantially free from sorbitic structure, and again drastically cooling.

9. The method of developing in` malleable cast iron, a grain having a homogeneous spheroidized cementite pearlitic structure, substantially free from sorbitic structure, and developing in 'the treated iron a strength of 87,000 pounds and upwards per square inch, and a Brinell hardness of 200 and upwards, which comprises subjecting malleabilized cast iron to a. temperature of about 1600 F. for a time sufficient to combine with associated carbon, then drastically cooling to form a martensitic structure, then reheating to about 1325 F. for a time suflicient to convert said martensitic structure into a-homgeneous spheroidized cementite pearlitic structure substantially free from sorbitic structure, and again drastically cooling. pearlitic structure, and again drastically cooling.

10. 'Ihe'method of developing in malleable cast ir0n,a grain having a homogeneous spheroidized cementite pearlitic structure, substantially free from sorbitic structure, and developing in the treated iron a strength of 87,000 pounds and upwards per square inch, and Brinell hardness of 200 and upwards, which comprises subjecting malleabilized cast iron to a temperature of about 1600 F. for not less than one hour, then drastically cooling to form a martensitic structure, then reheating to about 1325 F. .for not less than one hour to convert said martensitic structure into a homogeneous spheroidized cementite pearlitic structure substantially free from sorbitic structure, and again drastically cooling.

11. The method of developing in malleable cast iron, a grain having a homogeneous spheroidized cementite pearlitic structure, substantially free from sorbitic structure, and having a combined carbon content of from about 0.75 percent to about 0.85 percent, a strength of about 87,000 pounds and upwards per square inch and a Brinell hardness of about 200 and upwards, which comprises subjecting malleabilized cast iron to a temperature of about 1600 F. for a time sullcient to combine with associated carbon, then drastically cooling to form a martensitic structure, then reheating to about l325 F. for a time suiiicient to convert said martensitic structure into a homogeneous spheroidized cementite pearlitic structure substantially free from sorbitic structure, and again drastically cooling.

12.V The method of developing in malleable cast iron, a grain having a homogeneous spheroidized cementite pearlitic structure, and having a combined carbon content of from about 0.65 percent to about 0.35 percent, a strength of about 87,000 pounds and upwards per square inch and a Brinell hardness of about 200 and upwards, which comprises subjecting malleabilized east iron to a temperature of about 1600 F. for not less than one hour, then drastically cooling to form a martensitic structure, then reheating to about 1325 F. for not less than one hour to convert said martensitic structure into a homogeneous spheroidized cementite pearlitic structure, and again drastically cooling.

13. The method of developing in malleable cast iron, a grain having a homogeneous spheroidized cementite pearlitic structure, and having a combined carbon content of from about 0.55 percent to about 0.85 percent, a strength of about 80,000 pounds and upwards per square inch and a Brinell hardness of about 150 and upwards, which comprises subjecting malleabilized cast iron to a temperature of about 1600 F., in a carburizing environment, for a' time suilicient to combine with associated carbon,` then drastically cooling to form a martensitic structure, then reheating to not less than 1325" F. for a time suicient to convert said martensitic structure into a homogetensitic structure, then reheating to about 1325 F. fc'. not less than one hour to convert said martensitic structure into a homogeneous spheroidf' ized cementite pearlitic structure, and again drastically cooling.

15. A heat treated cast iron having a homogeneous spheroidized cementite pearlitic structure and substantially free from sorbitic structure, with a combined carbon contentl of from about 0.50 percent to about 0.85 percent, and a tensile strength of about 80,000 pounds and upwards per square inch.

16. A heat treated cast iron having a homogeneous spheroidized cementite pearlitic structure, with a combined carbon content of from about 0.50 percent to about 0.85 percent, a tensile strength of about 80,000 pounds and upwards per square inch, and a Brinell hardness of from about upwards.

17. A heat treated cast iron having a homogeneous spheroidized cementite pearlitic structure and with a carburized surface, said iron having a combined carbon content of from about 0.65 percent to about 0.85 percent, a tensile strength of about 87,000 pounds and upwards per square inch, and a Brinell hardness of from about 200 upwards. 'Y

18. A heat treated cast iron having a homogeneous spheroidized cementite pearlitic structure and substantially free from sorbitic structure,

with a combined carbon content of from about 'square inch, a Brinell hardness of from about 200 upwards, and an elongation of about nve percent, and upwards.

19. The method of heat treating malleable cast iron, which comprises heating the iron to a ternperature between 1475 F. and 1600 F. to form iron carbide, quenching said iron, and reheating to a temperature of about 1325 F. for a period of time suiicient to form a spheroidized structure.

20. A heat treated cast iron product containing a major portion of the carbon in graphitic form and having a microstructure in which the matrix consists chiefly of spheriodized or globular cementite.

21. In the heat treatment oi' white iron to develop therein a spheroidized cementitic structure and enhanced physical properties, the steps which comprise heating such iron to a temperature above the critical for a length of time sufficient to develop in such iron a dispersion of free carbon throughout a matrix of pearlitic cementite, cooling the iron at a rate to preserve the dispersion so obtained, thereafter subjecting the iron to a temperature below but near the critical for a length of time suiicient to develop such spheroidized cementitic structure in the presence of temper carbon, and cooling the iron to room temperature. y

22. An alloy formed from white cast iron by heat treatment above and below the critical in which the massive cementite originally occurring therein is in the form of temper carbon, dispersed throughout the alloy, and some of the carbon contained in the iron is in the form of spheroidized cementite dispersed through the alloy.

MAURICE G. JEWE'I'I. SAMUEL C. HARRIS. 

